In laser absorption spectroscopy with frequency modulation detection, a laser diode is current modulated at a high frequency. This results in the optical frequency of the laser being modulated at the same frequency as the current. It also causes light amplitude modulation at the same frequency. The frequency modulated light is emitted from the laser diode, passed through a target zone, which may or may not contain a gas or gases of interest and received at a detector, which contains a photo detector. The gas or gases of interest will have an absorption spectrum containing one or more lines or frequency bands in which light of that frequency is absorbed.
As the laser light frequency scans across the gas absorption lines, the absorption varies. The challenge in the art is to see the small amplitude change in light level caused by gas absorption as the laser wavelength is scanned across the gas line above the amplitude variations caused by the laser diode.
The method depends upon the nonlinear absorption change as the laser line scans across the Lorentzian absorption line. In one conventional method, harmonics of the modulation frequency are measured. The photo detector circuit will see second third, fourth, etc. harmonics of the laser modulation frequency caused by the nonlinear gas absorption. Laser amplitude modulation is dominated by the fundamental modulation frequency so it does not swamp out the relatively weak harmonics. In another conventional method, the laser is modulated at two frequencies, which is referred to as the "two tone method." Nonlinear absorption will mix these frequencies so the photodetector sees a frequency component, which is the difference between the two frequency components.
Common to all of these techniques is that the detecting circuit must select a particular frequency component and reject the rest. This is known as homodyne detection. In the art, this is done by taking a local oscillator at the required frequency and mixing it with the detected signal. The mixer will generate a d.c. or low frequency output, which is easy to isolate using a low pass filter. A detected signal containing frequency components w.sub.0, w.sub.1, w.sub.2, w.sub.3, etc is mixed with frequency component w.sub.0, which is taken directly from the current modulator for the laser diode. The dc output (w.sub.0 --w.sub.0) from the mixer is isolated with a low pass filter and the level of this signal provides an indication of the presence of a target gas in the target zone.
It is also known to simultaneously modulate the diode current at a relatively low frequency using a ramp. This ramp has a relatively large amplitude so it will scan the laser frequency through the absorption line. In this way it is not necessary to control the laser frequency so that it exactly coincides with the gas absorption line, which is difficult. The detected high frequency signal under these conditions is not at a d.c. frequency, but is modulated as the laser scans across the absorption line. This results in the well known "W" shaped detected waveforms.
In the art the required local oscillator is generated by taking the laser modulation signal and modifying it to give the desired local oscillator, as for example shown in Koch, U.S. Pat. No. 5,301,014, in which the second harmonic signal is detected. In this case the local oscillator is formed by taking the diode/laser modulator signal and passing it through a frequency doubling circuit. As a result the local oscillator has fixed amplitude and phase.
The use of a mixer to detect a chosen frequency is sensitive to phase. The mixed output is maximum when the signal and local oscillator are in phase and zero when they are 90.degree. out of phase. This is referred to as phase sensitive detection. This method is preferred because it results in high signal to noise ratio. The electrical random noise passing through a filter is proportional to the square root of the bandwidth so that a small bandwidth filter results in a low noise level. If the filter is tuned to the signal, it will have minimal effect upon the signal so that a narrow bandwidth filter will provide a high signal to noise ratio. It is, however, difficult to construct electrical filters with a high Q-value, which is the ratio of the signal frequency and bandwidth. However the mixing circuit used in phase sensitive detection shifts the signal frequency to a low value close to d.c. In this case it is possible to use a relatively low Q low pass filter and obtain a small bandwidth and random noise throughput.
Since phase sensitive detection depends upon the relative phases of the signal and local oscillator, these phases must be adjusted and then maintained. For fixed path length applications the phase of the signal is constant so that adjustment is usually performed using a phase shifting circuit in the local oscillator.